1. Field of the Invention
The present invention relates to a variable geometry optical gas detector. More specifically, the present invention relates to a gas detector of the non-dispersive optical type in which the electromagnetic radiation in the infrared band is emitted by a source for the purpose and channelled to an analyser through an optical path of variable geometry and configurable depending on the nature of the gas to be detected.
2. Description of the Related Art
In the prior art, detectors of gaseous substances are known of which function on the optical spectrophotometry principle by means of dispersive and non-dispersive sensors. These sensors detect the absorption of part of the electromagnetic radiation emitted by a source, passing through a gaseous substance appropriately interposed between said source and an analyser. The variation in the beam of electromagnetic radiation, due to the quantity of radiation absorbed by the gaseous substance, determines the nature and quantity of the latter.
The present invention relates to non-dispersive optical gas detectors which use electromagnetic radiation in the infrared band (NDIR Non Dispersive Infra Red). However, the same solution is also applicable to dispersion sensors and to sensors based on the absorption of electromagnetic radiation having wavelengths different from infrared.
The NDIR sensors detect the attenuation of the infrared (IR) luminous radiation caused by a target gas present in a gas sample. The degree of attenuation is a function of the absorption wavelength of the IR radiation, of the length of the optical path covered by the same, of the nature and concentration of the target gas present along said optical path and of the wavelength of the electromagnetic radiation detected by the analyser.
Normally the light source is provided by means of a filament bulb and the analyser by a pyroelectric sensor or by a thermopile.
It is also known that the level of sensitivity and accuracy of the detector is proportional to the length of the optical path between the source and analyser but it is desirable, for reasons of space and standard applications, to make a detector of reduced, compact dimensions.
It is, furthermore, advantageous to minimise the distance across which the gas is diffused to reach the optical path, so as to minimise the detection times of the detector.
Many examples of detectors are present in the state of the art and on the market. In general, NDIR detectors comprise a source of radiation, an analyser sensitive to the radiation emitted and an optical path which the radiation moves along in the passage from the source to the analyser. These detectors are also provided with means for allowing a sample gas, containing the target gas, to be introduced along the optical path. Said means may be passive, for example, holes through which the sample gas may spread, or active, for example micro pumps.
The analyser measures the intensity of the radiation of one or more wavelengths determined and corresponding to the absorption bands of the target gas and produces an electric measurement signal. Generally, said electric measurement signal in output from the analyser is compared to a reference signal relative to the intensity of the radiation on several wavelengths which are not absorbed by the target gas.
Some technical solutions for these NDIR detectors are described in the patents GB 2,317,010 and GB 2,368, 392. These embodiments use a tube as the optical path, with the source and analyser placed at ends of said tube, and means to allow the flow of sample gas through said tube.
Some solutions, described in GB 2,353,591 and U.S. Pat. No. 6,455,854 refer to detectors of larger dimensions which form an open optical path (without a tube) with the source and the analyser again placed at the ends of said optical path. All these types of detectors have the drawback however of being cumbersome and not utilisable where compact forms and dimensions are required.
The patent GB 2,396,405 describes a detector with a relatively long optical path, placed in a space limited by means of grooves which define a flat spiral path or thread. The source and the analyser are always placed at the ends of the optical path.
Other patents, such as GB 2,401,432, GB 2,372,099 and GB 2,369,884 describe detectors which use circular/spherical paths of compact dimensions and elements having optically fixed reflector surfaces such as in JP 2008 145292 and GB 2,316,172, again made with limited spaces and with the source and the analyser positioned at the end of the optical path and where, in the case of JP 2008 145292 the radiation is aimed towards the analyser at various angled trajectories. Especially in the case of GB 2,449,433, the optical path develops on two superposed levels and the optical radiation is deviated by appropriate means from one level to the other.
The typical drawback of this latter type of embodiment relates to the fact that the luminous radiation, not following a rectilinear path and on various levels, is considerably attenuated due to the refraction thereof on the delimiting and/or reflector surfaces of the optical path.
A further drawback of these detectors relates to the fact of not being adaptable to the various types of gas since each sensor has a fixed optical path with a specific length for a certain type of gas.